Sort comments in a Code review according to files and lines to which they are connected to and then according to a timestamp of a last edit instead of sorting it according to a timestamp of a last edit only.
Ideally indent comments to the same line and add a Reply button too.

When a changeset is removed the workspace is switching to the parent changeset and all changes get lost.
I think it is better to keep all changes in the "pending changes", so you do not lose any changes and you can add some more changes.

It often happens to me that I forget some small changes, so I have to check in again producing a lot of unnecessary changesets.

Usually, I don't need annotation of a whole file (which is really slow for big files); an annotation of a specific line or maybe several lines would be enough.
New context menu command Annotate this line from Diff/Merge tool would be very helpful.

LOCK a branch so no one can perform a merge or checkin except the one who took the lock.

Current scenario: Two or more developers are are working on child branches and want to merge back to the parent branch, if both initiate the merge, the first will succeed but the second will have to re-merge again. To avoid this, the developers can send an email, asking all to wait until he finishes the merge...
Proposed solution: Allow the one to take a lock on the target branch, do the work, and release it. During the lock, others should be restricted from accessing or doing actions on the lock (including an attempt to merge).

LOCK a branch so no one can perform a merge or checkin except the one who took the lock.

Current scenario: Two or more developers are are working on child branches and want to merge back to the parent branch, if both initiate the merge, the first will succeed but the second will have to re-merge again. To avoid this, the developers can send an email, asking all to wait until he finishes the merge...
Proposed solution: Allow the one to take a lock on the target branch, do the work, and release it. During the lock, others should be…

As mentioned in the following links, the changeset / branch Diff view from the Plastic GUI can be automatically opened to show a specified changeset or branch. This can be done either via command line / script or (on Windows) by a URL containing a special "Plastic://" protocol:

The History view and History as 2D tree view do not contain merges affecting the item and do not show any reverts. Therefore those views are not really a history, but only a list of different variants of the item existing in the history.
See comments for a deeper explanation.